Process for obtaining good acid wash color solvent xylenes



Oct. 22, 1957 R. M. LOVE PROCESS FOR OBTAINING GOOD ACID WASH COLOR SOLVENT XYLENES Filed Aug.

H H so DISTILLAT/ON gigs fi l2 2 4 coLumv EXTRACT/0N l5 (AG/D SETTLER mum /4 XYLENES A B ll 1a 'SLUDGE RAFF/NATE 25 HEBOILEI? PARAXYLENE PARAXYLENE M CRYSTALL/ZAT/O/V E J u/v/r 2/ saws/1r XYLENES INVEN TOR.

Roberf M. Love,

1 2,810,773 PROCESS FOR OBTAENING GGGD ACE) WASH COLOR SULVENT XYLENES Robert M. Love, Baytown, Ten, assignor, by mesne assignments, to Essa Research and Engineering Company,

Elizabeth, N. 3., a corporation of Delaware Application August 2, 1954, Serial 447,1(Bfi 2 Claims. (Ci. 260-674) The present invention is directed to a procedure for treating a mixture of C8 aromatics to obtain as product solvent xylenes having a good acid wash color.

Aromatic mixtures containing isomeric xylenes and ethylbenzene are available in commercial processes such as from the hydroforming of naphthas. These feed stocks contain appreciable quantities of olefins and paraffins. The present invention is directed to a procedure for recovering from such a mixture a solvent xylenes fraction having a good acid wash color.

Other objects and advantages of the present invention will be seen from the following description taken in conjunction with the drawing in which the sole figure is in the form of a diagrammatic flow sheet.

Turing now specifically to the drawing, a feed stock mixture containing Ca aromatics is charged through inlet line 11 to extraction unit A where it is solvent extracted in order to concentrate the aromatics portion of the feed stock. In extraction unit A, the feed stock may, for example, be contacted in conventional manner with a selective solvent, for example liquid sulfur dioxide, phenol or ethylene glycol, to produce an extract fraction of high aromatics content and a raifinate fraction of low aromatics content. Solvent is recovered from each of these fractions by suitable means, such as distillation, and is further used to treat additional portions of feed. In a preferred method of operation, liquid sulfur dioxide is employed as solvent, and the extract fraction is subsequently contacted with a high boiling parafiinic oil, known as wash oil, to further concentrate the aromatics content thereof. By way of illustration, a high boiling wash oil is shown passed into the extraction unit A through line 12 and a rafiinate fraction is shown withdrawn through line 13. It is to be understood that the showing of extraction unit A is illustrative only and that in practice any suitable type of solvent extraction procedure may be used for concentrating the aromatics component of the feed stock. From extraction unit A the extract compoent passes through line 14, has sulfuric acid added through inlet line 15 and passes to acid settler B where it is allowed to settle into two fractions under the influence of gravity. The acid treating step removes the olefins from the hydrocarbon fraction. From acid settler B the sludge is removed through line 16 and the olefin free fraction is removed through line 17, has admixed therewith a bottoms fraction from a subsequent distillation step by way of line 18 and the mixture is passed as feed to a first distillation column C.

In distillation column C the material charged thereto is separated into a bottoms fraction withdrawn through outlet line 19 and containing principally the wash oil which was picked up by the charge stock in extraction unit A, and an overhead fraction which consists of at least 90% mixed isomeric xylenes and ethylbenzene and is removed through line 20 and passed as feed to second distillation column D. In second distillation column D an overhead fraction is removed through line 21. This overhead fraction consists of at least 70% of the material charged to the second column and contains most of the paraxylene, metaxylene and ethylbenzene charged to the system. This fraction is particularly adaptable for use as a feed stock to a unit where paraxylene is recovered by crystallization and, accordingly, in the drawing is shown as charged to paraxylene crystallization unit E lfiatent C Patented Oct. 22, 1957 from which paraxylene is recovered as product through line 22. From column D bottoms side stream is with drawn through line 23. This bottoms side stream is a mixed aromatic fraction suitable for use commercially as a solvent xylenes fraction and contains from 14 to 25% of the charge and consists principally of orthoxylene. A bottoms fraction, consisting of from 1% to 5% of the charge to column D, is removed through bottoms line 24. In paraxylene crystallization unit E, the aromatics mixture charged thereto is subjected to low temperature crystallization and centrifugation to recover a paraxylene fraction which is removed as product by line 22. The filtrate recovered from the centrifugation is of good acid wash color and valuable for use as a solvent xylenes mixture and is withdrawn through line 29 and may be admixed with the bottoms side stream in line 23 for utilization as a solvent xylenes fraction.

The withdrawal of the bottoms fraction from column D insures that the solvent xylenes fraction withdrawn through line 23 is free from minute quantities of noxious compounds which may be formed in the distillation as described and which, if retained in the side stream 23, would cause an increase in acid wash color so that it would not meet conventional commercial specifications. The acid wash color referred to herein is defined as the color developed in the separated acid when a sample is agitated with sulfuric acid under theconditions prescribed in the test. It is determined according to the test method designated D848-47 by the American Society for Testing Materials (ASTM).

The bottoms side stream removed by line 23 from column D is removed at least one bubble cap tray above the bottom of the column. As shown in the drawing, column D is provided with a reboiler F into which bottoms line 24 discharges with vapors returned to column D through line 25, heat or the boiler being provided as indicated by heating coil 26. From reboiler section F, the fraction passes through line 18 for admixture with fresh charge in line 17 for the first distillation column C. The mentioned minute quantities of noxious compounds which cause an increase in acid wash color in the final fraction are removed in the bottoms fraction withdrawn from line 19 in column C and are discharged from the system.

At times it may be desirable to return the bottoms withdrawn through line 18 to acid treatment and if this is desired valve 18a in line 18 is closed and valve 27 in branch line 23 is opened so that the bottoms passes through line 28 and there is admixed with the extract from the extraction unit A in line 14 and the mixture mixed with acid through line 15 and then passes through the settler B. Sulfuric acid is selective for these noxious color producing compounds and combines with them to form acid sludge which is removed from the system.

If a Wash oil is not employed in the solvent extraction step, advantage may still be taken of the present inven tion. In that case, suflicient aromatics of nine carbon atoms or higher must be included with the crude xylenes in column C to provide at least about 5 percent of the total aromatics in the stream. In this case, the bottoms removed from tower C through line 19 consists essentially of C9 and heavier aromatic which can be employed for uses other than solvents, e. g. for inclusion in gasoline.

The procedure as described herein produces a fraction which is withdrawn as product through line 23 and which meets the ASTM standard specifications for 10 xylenes (designation D846-50), which includes an acid wash color not darker than No. 6 color standard. It is to be emphasized that if the bottoms fraction is not removed from column D as described, a satisfactory acid wash oil color fraction cannot be secured from column D. The bottoms fraction containing an appreciable amount of xylenes must be removed since the noxious acid wash color producing bodies which are essentially non-volatile are in too small a quantity to constitute a bottoms stream by themselves. Thus, some desirable components must be withdrawn in order to' remove the noxious compounds but by recycling the bottoms stream to the first distilla- I tion column the noxious components can bediscarded from the system as a bottoms stream together with a high boiling wash oil which enters the system during the extraction step. t

In order to illustrate further the practice of the present invention, the following typical examples of the I streams in the various portions of thesystem are given:

The feed stock charge intinlet line 11 may typically have a boiling range from 250 ;to 3003 R, a total content of isomeric. xylenes and ethylbenzene, within the a range of 40% to 60%, olefins in therange of 1% to 4% I Table I Components: 1 Amount percent Paraxylene r a 15.4 Metaxylene 40.0 Ethylbenzene 17.6 orthoxylene 20.4

Total xylenes and ethylbenzene 93.4 Other aromatics 5.3

Light .paraifins 1,3

A typical composition for the overhead fraction with-- drawn from second distillation column D is shown in Table II hereafter: I

I Table II I Components: Amount percent I Paraxylene I I 18.1 Metaxylene 46.4 Ethylbenzeue 21.0 Orthoxylene 11.0

Total xylenes 96.5 Other aromatics 2.7 Light parafiins .8

A typical composition ofthebottoms side stream re moved from column D through line 23 is shown in Table III hereafter: I

orthoxylene I Total mixed xylenes and ethylbenzene- 77.4 I

other-aromaticsuu' 18.6 Light parafiins 40 'It is to be mentioned that the overhead fraction reirig,.as shown in the typical example, over 18% paraxylene is a particularly valuable charge'stock for recovering paraxylene by low temperature crystallization centrifugation since'it allows a yield 'oftparaxylene'in excess of 11% to be obtained. 7

While specific examples have been given in describing the practice of the present invention, it is to be understood that these specific examples are given by way of illustration only and not by way of limitation. I

4 The invention claimed is: ,1..A method for recovering solvent xylenes of good acid wash color from an olefin free high xylenes content fraction containing the isomeric xylenes and ethylbenzene the aromatic content of which has been concentrated by solvent extraction with employment of 'a wash oil having a higher boiling range than said xylenes fraction including the continuously performed simultaneous steps of charging a stream of said high xylenes content fraction to a first distillation column and removing from said first distillation column a bottoms stream consisting-principally of wash oil and discarding ittfrom the system, removing from said first distillation column as an over- 7 head fraction a mixture'containing at least-90% mixed isomeric xylenes and ethylbenzene and charging" it as feed to a second distillation column, removing from said second distillation column an overhead fraction adapted to be further treated for the recovery ofparaxylene there? from consisting of at least 70% of the charge to the said second column and containing most of the araxylene, metaxyle'ne and ethylbenzene charged to the system, removing from said second distillation column a bottoms fraction consisting of from 1% to 5% of the charge to said second column and containing impurities formed in said distillation column, charging at least the major portion of said second distillation column bottoms fraction as a part of the high xylenes content fraction fed to said first column and'removing from said second column a bottoms side stream consisting of the remainderjof material charged to said second column and being a solventrx ylenes fraction containing a major portion of the orthoxylene in the charge to said second column and having a good acid wash color.

2. .A method for recovering solvent xylenes of good acid 'wash color from an olefin free high xylenes content fraction containing the isomeric xylenes and ethylbenzene the aromatic content, of which has been concentrated by solvent extraction including the continuously performed 7 simultaneous steps of charging a stream of said high xylenes content fraction containing at least about 5% by Weight of aromatic hydrocarbons-containing at least 7 9 carbon atoms to a first distillation column and removing from said first distillation column a bottoms stream consisting of at least about 5% of the charge to said first distillation column and containing substantially all of said aromatic hydrocarbons containing at least 9 car-' bon atoms and discarding it .from the system, removing from said first distillation column as an overhead fraction" a mixture containing at least 90% mixed isomeric xylenes Tablelll Components: I Amount percent Paraxylene 5 1.7 Metaxylene 6.9-

Ethylbenzene I 0.4

moved from second column'D through line21 contain-' and ethylbenzene and charging it as feed to a second distillation column, removing from said second distillation column an overhead fraction adapted to be further treated for the recovery of araxylene therefrom consisting of at least 70% of the charge to the. said second column and 7 containing most of the paraxylene, metaxylene, and eth-' ylbenzenecharged to the system, removing from said second distillation column a bottoms fraction consisting of from 1% to 5% of the charge to said second column and containing impurities formed in said second distillation column, charging at least the major portionof said second distillation column bottoms fraction as a part of the high xylenes content fraction fed to said first column and removing from said second column a bottoms side stream consisting of the remainder of material charged to said second column and being a solvent xylenes frac tion containing a major portion of the orthoxylene in the charge to said second column and having a good acid' wash color. I

References Cited in the file of this'patent 0 UNITED STATES PATENTS 2,407,820 2,532,276 Birch et al7 Dec; 5,1950 2,533,232 Dressler Dec. 1-2, 1950 I Durrum Sept." 17, 1946 

1. A METHOD FOR RECOVERING SOLVENT XYLENES OF GOOD ACID WASH COLOR FROM AN OLEFIN FREE HIGH XYLENES CONTENT REMOVING FROM SAID FIRST DISTILLATIN COLUMN AS AN OVERHEAD FRACTION A MIXTURE CONTAINING AT LEAST 90% MIXED ISOMERIC XYLENES AND ETHYLBENZENE AND CHARGING IT AS FEED TO A SECOND DISTILLATION COLUMN, REMOVING FROM SAID SECOND DISTILLATION COLUMN AN OVERHEAD FRACTION ADAPTED FRACTION CONTAINING THE ISOMERIC XYLENES AND ETHYLBENZENE THE AROMATIC CONTENT OF WHICH HAS BEEN CONCENTRATED BY SOLVENT EXTRACTION WITH EMPLOYMENT OF A WASH OIL HAVING A HIGHER BOILING RANGE THAN SAID XYLENES FRACTION INCLUDING THE CONTINUOUSLY PERFORMED SIMULTANEOUS STEPS OF CHARGING A STREAM OF SAID HIGH XYLENES CONTENT FRACTION TO A FIRST DISTILLATION COLUMN AND REMOVING FROM SAID FIRST DISTILLATION COLUMN A BOTTOMS STREAM CONSISTING PRINCIPALLY OF WASH OIL AND DISCARDING IT FROM THE SYSTEM, TO BE FURTHER TREATED FOR THE RECOVERY OF PARAXYLENE THEREFROM CONSISTING OF AT LEAST 70% OF THE CHARGE TO THE SAID SECOND COLUMN AND CONTAINING MOST OF THE PARAXYLENE, METAXYLENE AND ETHYLBENZENE CHARGED TO THE SYSTEM, REMOVING FROM SAID SECOND DISTILLATION COLUMN A BOTTOMS FRACTION CONSISTING OF FROM 1% TO 5% OF THE CHARGE TO SAID SECOND COLUMN AND CONTAINING IMPURITIES FORMED IN SAID DISTILLATION COLUMN, CHARGING AT LEAST THE MAJOR PORTION OF SAID SECOND DISTILLATION COLUMN BOTTOMS FRACTION AS A PART OF THE HIGH XYLENES CONTENT FRACTION FED TO SAID FIRST COLUMN AND REMOVING FROM SAID SECOND COLUMN A BOTTOMS SIDE STREAM CONSISTING OF THE REMAINDER OF MATERIAL CHARGED TO SAID SECOND COLUMN AND BEING A SOLVENT XYLENES FRACTION CONTAINING A MAJOR PORTION OF THE ORTHOXYLENE IN THE CHARGE TO SAID SECOND COLUMN AND HAVING A GOOD ACID WASH COLOR. 